Transfer project from GitLab 11.4 to 16.0

Is it possible to export a project from gitlab 11.4 self managed instance and import it to another gitlab 16 self managed instance? if yes, then how so?

Have you simply tried to export the project in the GUI (Settings → General → Advanced → Export) and import into the new one?

I haven’t tried that (have done export-import a couple of times, but only with versions close to each other), but at least the repository itself has not changed format, so I don’t see why that should get transferred.
But as @balonik says: Try it, if you get any errors, the error message probably says what part, then my instinct would be to delete that part of the export (the format of which is quite simple), or perhaps just empty it (there might be something that depends on it being there).

There is no option to export issues in 11.4
Is it possible to export issues, wiki pages etc as well?

I doubt that. I don’t have any documentation for it, but to my knowledge exporting projects is a very ld feature, and the first time I used it was around the time was 11.4 was current.
The feature is now (and I think it was the same back then) located Settings (for the project)/General/Advanced. Both issues and wiki pages and mentioned among the things that are exported on new versions, as I don’t believe those were new features around 11.4, there were probably exported back then too.

Sorry, I didn’t have permission to export at that time.
But I have tried it now and it works for the repo files but not for issues and wiki.
The .tar.gz export file does contain all the issues in a json file and a wiki bundle file, but for some reason they are not imported in GitLab 16. Also no error is thrown.
Is there any other way to do this? Can I manually upload issues using the json file and wiki using the bundle file?
There is a separate option to import issues but it takes csv input file.
Thanks for you help so far.